SrFe0.5Ru0.5O2: Square-Planar Ru2+ in an Extended Oxide - Journal

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SrFe Ru O : Square Planar Ru in an Extended Oxide Fabio Denis Romero, Steven Burr, John E. McGrady, Diego Gianolio, Giannantonio Cibin, and Michael A. Hayward J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 17 Jan 2013 Downloaded from http://pubs.acs.org on January 17, 2013

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Journal of the American Chemical Society

SrFe0.5Ru0.5O2 : Square-planar Ru2+ in an Extended Oxide Fabio Denis Romeroa, Steven J. Burra, John E. McGradya, Diego Gianoliob, Giannantonio Cibinb and Michael A. Haywarda* a

Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, United Kingdom. b Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom ABSTRACT: Low-temperature topochemical reduction of the cation disordered perovskite phase SrFe0.5Ru0.5O3 with CaH2 yields the infinite layer phase SrFe0.5Ru0.5O2. Thermogravimetric and X-ray absorption data confirm the transition metal oxidation states as SrFe0.52+Ru0.52+O2, thus the title phase is the first reported observation of Ru2+ centers in an extended oxide phase. DFT calculations reveal that while the square-planar Fe2+ centers adopt a high-spin S = 2 electronic configuration, the square-planar Ru2+ cations have an intermediate S = 1 configuration. This combination of S = 2, Fe2+ and S = 1, Ru2+ is consistent with the observed spinglass magnetic behavior of SrFe0.5Ru0.5O2.

Introduction Complex transition metal oxides have been the subject of enduring interest due to the wide variety of physical properties they exhibit. These range from correlated electronic behavior such as superconductivity 1 or magnetoresistance,2 to the electrochemical and transport behaviors exploited in their use as electrodes and electrolytes in fuel cells 3 and lithium ion batteries,4 to their use as heterogeneous catalysts in a number of industrially important processes.5 The broad range of physical properties exhibited by complex transition metal oxide phases can be attributed to the presence of electrons in partially filled d-states, which interact strongly with each other via the extended metal-oxide lattice present in these materials. As a consequence the electronic structure of such materials depends on the transition metal coordination geometry and valence electron count, which together define the local transition metal electronic configuration, and the long range metal-anion lattice structure, which defines the nature and strength of the electronic coupling between metal centers. By controlling these features of extended solids, we can tune the electronic structure of materials and thus their physical behavior. Conventional high-temperature synthesis routes allow the preparation of a wide range of complex oxide phases. However, although this synthesis approach is widely employed, it is necessarily limited to the preparation of only the most thermodynamically stable phases in any composition range, precluding the formation of a wide range of metastable materials. By employing low-temperature ‘soft’ synthesis techniques, which operate under conditions where kinetic rather than thermodynamic considerations dominate product selection, the range of preparable complex oxides can be extended. Recently lowtemperature topochemical reduction reactions, utilizing binary metal hydrides as reducing agents, have allowed the preparation of complex oxide phases containing transition metal centers in highly unusual oxidations states and/or coordination geometries, such as square-planar Ni1+ and Co1+ and octahedral Mn1+.6-9 Using this strategy Tsujimoto et al recently reduced the cubic perovskite phase SrFeO3-δ to SrFeO2.10 This Fe2+ oxide

adopts an infinite layer structure consisting of sheets of apexlinked Fe2+O4 square-planes stacked with Sr2+ cations (Figure 1). This structure type is adopted by only a small number of complex oxide phases, such as LnNiO2 (Ln = La, Nd) 6,7 and Sr1-xCaxCuO2,11 which tend to have local transition metal valence electron counts (Ni1+, Cu2+ = d9) which help to stabilize the square-planar transition metal coordination geometry. It is therefore somewhat surprising that SrFeO2 adopts an infinite layer structure, as the Fe2+ centers in this phase have a d6 valence electron count which adopt a high-spin, S = 2, electronic configuration at ambient pressure,12,13 providing no obvious energetic stabilization for a square-planar coordination geometry. However, even though there is no obvious stabilizing mechanism, it is clear that the d6 electron count does play a significant role in directing the reduction reaction to form an infinite layer structure, as partially substituted SrFe1-xMxO3-δ phases (M = Co, Mn) do not form infinite layer phases on reduction when x > 0.3.14 In addition the infinite layer structure of the d6, Fe2+ phase SrFeO2 and the related RuddlesdenPopper phase Sr3Fe2O5 are remarkably robust, exhibiting relatively high decomposition temperatures.10,15 In this study we utilize the robustness of the SrFeO2 framework to stabilize Ru2+ cations within an extended oxide framework for the first time.

Figure 1. Topochemical reduction of SrFeO3 yields the infinite layer phase SrFeO2.

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Experimental Sample preparation. Samples of SrFe0.5Ru0.5O3 were prepared via a citrate gel method. Suitable stoichiometric ratios of SrCO3 (99.994 %), Fe2O3 (99.99 %) and RuO2 (99.99 %, dried at 800ºC) were dissolved in a minimum quantity of 6 M nitric acid. 1.333 mole equivalents of citric acid and 5 ml of analar ethylene glycol were added and the solution heated with constant stirring. The gels thus formed were subsequently ground into a fine powder, placed in an alumina crucible and heated at 1 °C min-1 to 900 °C in air. The powders were then reground, pressed into 13 mm pellets and then heated to 1300ºC in air for 2 periods of 2 days. Attempts to prepare SrFe1-xRuxO3 phases with x ≠ 0.5 were unsuccessful, resulting in the formation of mixtures of SrRuO3 and SrFeO3-x. Samples of SrRuO3 were prepared via a ceramic synthesis method.16 Suitable stoichiometric ratios of SrCO3 (99.994) and RuO2 (99.99 %, dried at 800ºC) were ground together in an agate pestle and mortar, pressed into pellets and heated in air for two periods of two days at 1150 ºC. All samples were observed to be single phase by X-ray powder diffraction with lattice parameters in good agreement with literature values.16,17 Samples of SrFe0.5Ru0.5O2 were prepared by reducing SrFe0.5Ru0.5O3 with 2 mole equivalents of CaH2 at 400 ºC for 2 periods of 48 hours in a ‘venting’ apparatus described previously.18 After reaction, calcium containing phases were removed from samples by washing with 4 × 100 ml aliquots of 0.1M NH4Cl in methanol and then a further 4 × 100 ml aliquots of clean methanol before being dried under vacuum. Reactions between SrRuO3 and two mole equivalents of CaH2 were performed in sealed silica tubes. At reaction temperatures below 325ºC, no reaction was observed by X-ray powder diffraction. At reaction temperatures of 325ºC and above samples decomposed to form mixtures of SrO, CaO and elemental ruthenium. Characterization. X-ray powder diffraction data were collected from samples contained in gas-tight sample holders using a PANalytical X’Pert diffractometer incorporating an X’celerator position sensitive detector (monochromatic Cu Kα1 radiation). Neutron powder diffraction data were collected using the D2B instrument (λ = 1.59Å) at the ILL neutron source, from samples contained within vanadium cans sealed under argon with indium washers. Rietveld profile refinements were performed using the GSAS suite of programs.19 Magnetization data were collected using a Quantum Design MPMS SQUID magnetometer. Thermogravimetric data were collected from powder samples under flowing oxygen using a Netzsch STA 409PC balance. X-ray absorption experiments were performed at B18 beamline of Diamond Light Source.20 The measurements were carried out using the Pt-coated branch of collimating and focusing mirrors, a Si(111) double-crystal monochromator and a pair of harmonic rejection mirrors. The size of the beam at the sample position was approximately 600 µm (h) x 700 µm (v). XANES data were collected at the Fe Kedge (7112 eV) in transmission mode with ion chambers before and behind the sample filled with appropriate mixtures of inert gases to optimize sensitivity (I0: 300mbar of N2 and 700 mbar of He resulting in an overall efficiency of 10% , It: 150mbar of Ar and 850 mbar of He, with 70% efficiency). The spectra were measured with a step size equivalent to 0.25 eV. Data were normalized using the program Athena 21 with a linear preedge and polynomial postedge background subtracted from the raw ln(It/I0) data. The samples were prepared in

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the form of a self supported pellet, with the thickness optimized to obtain an edge jump close to one. Calculations Density functional theory calculations were performed with the Siesta 2.6.8-LDAU22,23 package using a 6x6x10 k-point grid. The PBE functional24 was used throughout with Hubbard U values of 4.0 and 3.0 eV for Fe and Ru, respectively. The unit cell parameters were optimized to a tolerance of 0.005 eV/Å. The basis sets were of doublezeta+polarization quality, with core electrons described by norm-conserving pseudopotentials.25 Sr semi-core levels (4s, 4p) were included in the valence space. Magnetic couplings were computed by mapping the difference between high-spin and broken-symmetry determinants onto the diagonal elements of Heisenberg-type spin Hamiltonians incorporating only nearest-neighbor interactions. Full details are given in the supporting information. Results Structural and Chemical Characterization. Thermogravimetric data collected during the reoxidation of SrFe0.5Ru0.5O3-x to SrFe0.5Ru0.5O3 (confirmed by powder X-ray diffraction) are consistent with a composition of SrFe0.5Ru0.5O2 for the reduced phase, as shown in Figure 2. Neutron powder diffraction data collected at room temperature from SrFe0.5Ru0.5O2 could be readily indexed using a primitive tetragonal unit cell (a = 3.99 Å, c = 3.49 Å). Given the similarity between the lattice parameters of SrFe0.5Ru0.5O2 and SrFeO2,10 a structural model based on the infinite layer structure of SrFeO2, with a disordered 1:1 mixture of iron and ruthenium on the B-cation site, was refined against the neutron powder diffraction data collected from SrFe0.5Ru0.5O2. The structural refinement converged readily with a good statistical fit. Close inspection of the diffraction data revealed a series of weak additional diffraction features corresponding to a small amount of CaO (2.4(2) weight percent) not removed by the washing process, and so CaO was added to the structural model as a secondary phase to account for these features. A complete description of the refined structure of SrFe0.5Ru0.5O2 is given in Table 1. Figure 3 shows a plot of the observed and calculated diffraction data from the structural refinement.

Figure 2. Thermogravimetric data collected during the reoxidation of SrFe0.5Ru0.5O2 to SrFe0.5Ru0.5O3.


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Figure 3. Observed, calculated and difference plots from the refinement of SrFe0.5Ru0.5O2 against neutron powder diffraction data collected at room temperature. Lower tick-marks indicate peak positions for majority phase, upper tick-marks for the CaO secondary phase. Atom

x

y

z

Fraction

Uiso (Å2)

Sr

½

½

½

1

0.0041(2)

Fe/Ru

0

0

0

0.5/0.5

0.0040(1)

O

½

0

0

1

0.0061(2)

SrFe0.5Ru0.5O2 P4/mmm a = 3.9903(1)Å, c = 3.4978(1) 97.6(2) weight percent CaO Fm-3m a = 4.8152(2)Å 2.4(2) weight percent χ2 = 2.63, wRp = 5.62%, Rp = 4.37%

Table 1. The structure of SrFe0.5Ru0.5O2 refined against neutron diffraction data collected at 298 K. The oxygen stoichiometry of the SrFe0.5Ru0.5O2 reduced phase defines the average transition metal oxidation state as M2+, however it does not unambiguously define the individual oxidation states of the iron and ruthenium cations. In order to resolve this ambiguity, X-ray absorption spectra were collected from the iron K-edges of SrFe0.5Ru0.5O2, SrFeO2 and Sr2Fe2O5 as shown in Figure 4. These data reveal that the iron absorption edges of SrFe0.5Ru0.5O2 and SrFeO2 are almost identical, indicating that the iron centers in these two phases are in the same oxidation state and coordination environment (square-planar Fe2+), and are distinctly different from the absorption edge of the Fe3+ phase Sr2Fe2O5. Thus we can unambiguously define the transition metal oxidation states in the reduced material as SrFe2+0.5Ru2+0.5O2. To the best of our knowledge this is the first time Ru2+ centers have been observed in an extended oxide phase.

Figure 4. Normalized X-ray absorption spectra collected from the iron K-edge of SrFe0.5Ru0.5O2, SrFeO2 and Sr2Fe2O5.

Magnetic Characterization. Magnetization data collected from SrFe0.5Ru0.5O2 as a function of temperature in an applied field of 100 Oe (Figure 5) show a weak divergence between zero-field cooled and field cooled responses in the temperature range 75 < T/K < 300. Approximate fits to the Curie-Weiss law (χ = C/(T-θ) + K) in this temperature range yield values of C = 0.369(1) cm3 K mol-1, θ = -12.3(9) K, K = 0.00356(1) cm3 mol-1. The combination of a divergence between zero-field cooled and field cooled data and a small susceptibility indicates that there is strong coupling between magnetic centers even at high temperature preventing the spin-states of the Fe2+ and Ru2+ cations being deduced from the magnetization data. Below a local maximum in the zero-field cooled data at 60 K, the zero-field cooled and field cooled data diverge much more strongly, indicative of a magnetic transition. A magnetization-field isotherm collected from SrFe0.5Ru0.5O2 at 5 K (Figure 5) exhibits weak hysteresis and is displaced significantly from the origin, indicating that the ordering event at T ~ 60 K corresponds to the freezing of a spin glass. Neutron powder diffraction data collected from SrFe0.5Ru0.5O2 at 5 K show no evidence for long range magnetic order.



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considered here. As a result we have imposed no restrictions on the lattice parameters (i.e. a ≠ b ≠ c) in the optimization procedures. Of the three structures, the rocksalt form is the most stable, although column and planar alternatives lie only 0.05 and 0.07 eV higher in energy (energies given per SrFe0.5Ru0.5O2 formula unit). The relative thermodynamic stability of the rocksalt structure does not, however, have any direct bearing on the cation disorder as the arrangement of cations is inherited from the SrFe0.5Ru0.5O3 perovskite parent structure. The local electronic structure at Fe and Ru proves to be independent of the cation the ordering, and so we describe in detail below only the rocksalt configuration. Details of the column and planar alternatives are collected in the Supporting Information.

Figure 6. Optimized unit cells for the rocksalt, column and planar configurations of SrFe0.5Ru0.5O2.

Figure 5. Zero-field cooled and field cooled magnetization data (top) and magnetization-field isotherms (bottom) collected from SrFe0.5Ru0.5O2.

Electronic structure The electronic structure of SrFeO2 has been described independently by both Xiang et al.13 and Pruneda et al.,12 who showed that the Fe2+ centers adopt S = 2 configurations with the d z 2 orbital being doubly occupied. Strong intra-layer antiferromagnetic coupling between the iron centers, as anticipated based on the Goodenough-Kanamori rules,26 then imposes an antiferromagnetic ground state. We have conducted a parallel series of calculations on SrFe0.5Ru0.5O2 using a density functional approach (GGA + U).27 X-ray and neutron powder diffraction data reveal that the iron and ruthenium centers are completely disordered within the ABO2 framework. In order to model the different possible inter-cation interactions present in the cation disordered phase, we have considered three distinct limiting Fe/Ru ordered configurations (rocksalt, column and planar shown in Figure 6) arranged within an a’ = √2a, c’ = 2c expanded unit cell. Whilst the disordered structure can be refined with a tetragonal unit cell (P4/mmm spacegroup), it remains possible that Jahn-Teller instabilities at either Fe or Ru could induce orthorhombic distortions in the three limiting ordered structures

In the most stable electronic state of the rocksalt structure the intra-layer Fe-O-Ru coupling is ferromagnetic (J1 = -2.39 meV) while the inter-layer coupling is weakly antiferromagnetic (J2 = 1.00 meV). The partial density of states (PDOS) for Ru shown in Figure 7 confirms that the majority-spin d z 2 ,

d xz , d yz and d x 2 − y 2 levels (α on Ru1, β on Ru2) lie below the Fermi level, as do minority-spin d yz and d z 2 . Note that the x and y axes are defined to lie along the unit cell vectors, as a result of which it is the d xy orbital on each metal that is strongly O-M-O antibonding and not the d x 2 − y 2 as in the axis system used by Pruneda et al.12 In contrast, minority-spin d xz and d x 2 − y 2 , along with both spin components of d xy , lie above the Fermi level. The local configuration at Ru2+ is therefore a triplet, ( d z 2 )2( d xz , d yz )3( d x 2 − y 2 )1( d xy )0, consistent with Mulliken spin densities of ±2.20 for Ru1 and Ru2, respectively. In the Fe manifold, in contrast, all five components of the majority-spin manifold (α on Fe1, β on Fe2), are occupied, along with the minority-spin d z 2 , giving a high-spin quintet

configuration,

( d z 2 )2( d xz , d yz )2( d x 2 − y 2 )1( d xy )1

(Mulliken spin densities = ± 4.08 ), very similar to that in the


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parent SrFeO2 phase.12,13 The ( d xz , d yz )3 configuration at ruthenium drives a small but distinct orthorhombic distortion (optimized lattice parameters: a’ = 5.83 Å, b’ = 5.68 Å, c’= 6.58 Å, a’/b’ = 1.03), although the disorder in the crystal structure precludes the detection of such a small effect experimentally. The optimized cell parameters for the column and planar configurations are similar to those for the rocksalt analogue (a’ = 5.82 Å, b’ = 5.68 Å, c’= 6.63 Å and a’ = 5.81 Å, b’ = 5.69 Å, c’= 6.58 Å, respectively). The PDOS and Mulliken spin densities also confirm that the local electronic structure is independent of the precise arrangement of ions in the lattice: the S = 2 ( d z 2 )1( d xz , d yz )2( d x 2 − y 2 )1( d xy )1 and S = 1 ( d z 2 )2( d xz , d yz )3( d x 2 − y 2 )1( d xy )0 configurations are common to Fe and Ru, respectively, in all cases (supporting information, Figures S2 and S3).

Figure 7. Computed partial density of states (PDOS) for the antiferromagnetic ground state of the rocksalt configuration of SrFe0.5Ru0.5O2. Ru and Fe PDOS are shown on the left and right, respectively. Majority and minority-spin PDOS are shown as full and dashed lines, respectively.



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d xz , d yz )3 exchange. The very similar structure of the Ru-ODiscussion Reduction of SrFe0.5Ru0.5O3 with CaH2 leads to the formation of the infinite-layer phase SrFe0.5Ru0.5O2, in a process analogous to the reduction of SrFeO3 to SrFeO2.10 This behavior is in strong contrast to the reduction of SrRuO3, which is converted directly to elemental ruthenium and SrO, demonstrating that the presence of iron helps to stabilize the low oxidation states of ruthenium. Presumably this stabilization occurs because the extended oxide lattice couples the further reduction of the Fe2+ and Ru2+ centers - Ru2+ cannot be reduced to elemental ruthenium without destroying the extended oxide framework - so the endothermic reduction potential of iron can present a thermodynamic barrier to the further reduction of ruthenium centers which is not present during the reduction of SrRuO3 (Fe2+/Fe Eº = -0.447V; Ru2+/Ru Eº = +0.455V).28 The reduction behavior of SrFe0.5Ru0.5O3 is also rather different from that of substituted SrFe1-xMxO3 (M = Mn, Co; x > 0.3) phases, which are reduced to materials with noninfinite layer structures.14 This difference clearly shows that the presence of ruthenium cations does not disrupt the structural selectivity of the low-temperature reduction reactions, reinforcing the idea that a d6 transition metal electron count plays an important role in directing them to form products with square-planar transition metal coordination geometries.29 Four coordinate Ru2+ centers such as those seen in SrFe0.5Ru0.5O2 are relatively rare, even in molecular complexes, due to the inherent stability of 6-coordinate S = 0 systems. It is possible to prepare 4-coordinate Ru2+ complexes by utilizing macrocyclic or chelating ligands. These compounds tend to exhibit distorted ‘butterfly-shaped’ configurations which are stabilized by the adoption of low-spin, S = 0, configurations at the ruthenium centre.29 This ‘butterfly’ distortion can be sterically suppressed by employing bulky pincer-type ligands, resulting in the formation of square-planar complexes with either S = 1 or S = 0 spin configurations depending on the πdonating properties of the ligand set.30 The Ru2+ centers present in SrFe0.5Ru0.5O2 are observed to be square-planar by diffraction, as defined by the P4/mmm spacegroup symmetry (excepting any local orthorhombic local distortion), indicating that the packing requirements of the surrounding Sr-Fe-O lattice obstruct the local ‘butterfly’ distortions of the Ru2+ centers. The magnetic behavior of SrFe0.5Ru0.5O2, discussed below, and calculations described above, indicate the ruthenium centers adopt an S = 1 intermediate spin state within the square-planar coordination sites of SrFe0.5Ru0.5O2, consistent with the modest π-donating ability of the oxide ligands. Magnetic behavior. The parent SrFeO2, exhibits antiferromagnetic order below 473 K which is attributed to strong antiferromagnetic superexchange within the Fe-O-Fe layers {Fe( d xy )1-O(2p)-Fe( d xy )1} and weaker direct exchange between the layers {Fe( d xz , d yz )2-Fe( d xz , d yz )2} (note axis choice as described above).12,13 In the rock-salt configuration of SrFe0.5Ru0.5O2, in contrast, the intra-layer Ru-O-Fe coupling is ferromagnetic (J1 = -2.39 meV, see supporting information) because the Ru-based d xy orbital is now empty {Fe( d xy )1O(2p)-Ru( d xy )0}. The interlayer coupling, however, is antiferromagnetic (J2 = 1.00 meV) due to direct Fe( d xz , d yz )2-Ru(

Fe units in the column configuration also leads to ferromagnetic coupling within the layers (J1 =- 2.75 meV). In the planar alternative, however, strong σ-type {Fe( d xy )1-O(2p)-Fe( d xy )1} superexchange imposes antiferromagnetic ordering within the Fe-O-Fe layers (J1 = 1.73 meV), precisely as in SrFeO2. Antiferromagnetic ordering also prevails in the Ru-O-Ru layers (J1 = 4.77 meV), in this case due to π-type {Ru( d xz )1O(2p)-Ru( d xz )1} superexchange along the a’ lattice direction. The combination of ferromagnetic Fe-O-Ru (rock-salt and column) and antiferromagnetic Fe-O-Fe and Ru-O-Ru intralayer couplings (planar) inevitably leads to magnetic frustration in any microdomain where an Ru-O-Fe unit lies directly above either Ru-O-Ru or Fe-O-Fe (Figure 8), irrespective of the sign of the inter-layer coupling. Magnetization data collected from SrFe0.5Ru0.5O2 show a strong divergence between zero-field cooled and field cooled data at 60 K, which the strongly displaced magnetization-field isotherm collected at 5 K reveals to be a spin-glass freezing transition (Figure 5). The observed spin-glass behavior is therefore a natural consequence of the disorder and the different intra-layer couplings.

Figure 8. The frustrated ferromagnetic (FM) and antiferromagnetic (AFM) coupling interactions in a local Fe3Ru microdomain present in SrFe0.5Ru0.5O2.

Conclusion By utilizing the surrounding robust Sr-Fe-O framework, we have managed to impede the normally facile reduction of Ru2+, and prepare SrFe0.5Ru0.5O2, the first extended oxide phase to contain Ru2+ centers. Magnetization data supported by calculations indicate the square-planar Ru2+ centers adopt an S = 1 intermediate spin state, compared to an S = 2 spin state for the Fe2+ centers, consistent with the larger radial extent and hence stronger orbital-ligand interaction of 4d versus 3d metal orbitals. Given that the isostructural all-iron phase SrFeO2 undergoes a spin-state transition from S = 2 to S = 1, on the application of ~50 GPa pressure accompanied by a phase transition from an antiferromagnetic insulating state to a ferromagnetic metallic state,31 it seems likely that the replacement of half the S = 2, Fe2+ centers with S = 1, Ru2+ centers will significantly lower the pressure required for the analogous transition in SrFe0.5Ru0.5O2, potentially moving it into the technologically useful range.


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ASSOCIATED CONTENT Supporting Information. Detailed descriptions of the electronic structure for SrFeO2 and the column and planar forms of SrFe0.5Ru0.5O2.This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Tel : +44 1865 272623. Fax: +44 1865 272690. email: [email protected] Author Contributions The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT We thank E. Suard for assistance collecting the neutron diffraction data.

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Figure 1 38x18mm (300 x 300 DPI)



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Figure 4 141x243mm (300 x 300 DPI)


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Figure 5 197x358mm (300 x 300 DPI)



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Figure 7 156x139mm (300 x 300 DPI)



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TOC Figure 31x11mm (300 x 300 DPI)


SrFe0.5Ru0.5O2: Square-Planar Ru2+ in an Extended Oxide - Journal

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